Heating Up Magnetic Memory

Heating Up Magnetic Memory

Laser lollipop: The lollipop-shaped device shown in the scanning-electron micrograph at top is an optical antenna made of gold. Measuring 50 nanometers across at its widest part, it is part of a prototype magnetic-storage system being developed by Seagate. When added to the magnetic-data writing head of a hard disk, shown below, it couples laser light to tiny spots on magnetic-storage media.

Demand for data storage keeps going up, even while consumers expect the cost per bit to keep going down. However, the magnetic-recording materials used in today’s hard disks are reaching their storage limits and will probably max out within five years. To compete with newer technologies such as flash, the companies that make them need something new.

Now researchers at Seagate have demonstrated the feasibility of a new technology that could extend the capacity of magnetic data recording for many years more. Called heat-assisted magnetic recording, it involves blasting the magnetic regions of a disk with heat to make it possible to use more stable recording media. It should make it possible to record data at densities 50 times greater than will be possible when today’s technologies reach their limits.

“Within a few years, the magnetic-recording industry is going to have to find a new way forward,” because the materials currently used are nearing their physical limits, says Randall Victora, a professor of electrical and computer engineering at the University of Minnesota.

The hard drive inside most computers is made up of one or more spinning disks coated with a magnetically sensitive film consisting of tiny, jagged grains. Data is recorded when a small head moves over the disk, flipping the magnetization of one of these grains so that it either points up or down, for a 1 or a 0.

“As we make the storage density greater, we have to make the grains smaller,” says Ed Schlesinger, head of the department of electrical and computer engineering at Carnegie Mellon University, in Pittsburgh. “But you reach a point where the grains get so small, they become unstable,” and their magnetic state can be altered by small temperature fluctuations.

The problem cannot be overcome simply by switching to more stable recording media because today’s recording heads can’t write to them. So Seagate has been developing magnetic-recording heads that integrate a heating element. Blasting more magnetically stable grains with a short pulse of heat makes it much easier to flip them. When the media cools down again, the data is “frozen.”

Heat-assisted magnetic recording still presents a tremendous scientific and engineering challenge, though. The heat is provided by a rapid laser blast that must be focused down to a spot the size of an individual grain–less than 100 nanometers in diameter. This is impossible to do using conventional optics. Instead, it requires a new generation of optics that work in what’s known as the near field. The Seagate technology uses optical antennas, which can focus light energy onto areas smaller than any lens-based instrument can.